Many physical objects, particularly those having defined surfaces that are relatively wider than they are thick, are characterized by a resonant frequency, that is, a frequency at which those physical objects exhibit a maximum (or at least a local maximum) energy response to vibrations. For example, a wooden tabletop might, depending on its size, thickness, and the substance from which it is made, have a particular frequency at which it might vibrate, and at which it might amplify vibrations if those vibrations are applied to that tabletop at that frequency.
This can have the effect that a speaker, or a speaker in combination with an amplifier, can deliberately cause the tabletop to vibrate at a relative maximum, by emitting vibrations (such as sound) that match the resonant frequency of that tabletop.
This can also have the effect that a sensor, or a sensor in combination with an amplifier, can detect or otherwise determine the resonant frequency of a tabletop on which it sits, in response to vibrations of the tabletop and in response to whether that sensor detects relative amplification or relative damping of those vibrations.